Eastern Bluebirds chicks, like all organisms, are affected by the environment around them. I wanted to see how the quality of the soil around the nest and the quality of the bugs the chicks were eating affected their health, growth rate, the size of the brood, the number of broods produced in a year, and the sex ratio of the brood. In order to do this, I analyzed the Carbon, Nitrogen, and Phosphorus content of soil and bug samples taken from the area around the nest box. Carbon, Nitrogen, and Phosphorus are all essential nutrients. Carbon is the basic building block of matter; Nitrogen is used in the production of amino acids, proteins, and enzymes; and Phosphorus is necessary for DNA and ATP, or useable energy.
The ratios between Carbon, Nitrogen, and Phosphorus are typically used in assessing nutrient content. Nitrogen and Phosphorus are both limiting reagents, while Carbon is readily found in the environment, so low C:N and C:P ratios are preferable and more nutrient rich. High N:P ratios mean that Phosphorus is limited and low N:P ratios indicates that Nitrogen is limited.
I found that soil quality had very little direct impact on Bluebird chicks, while the quality of the food had a much more noticeable influence. Chick condition increased with an increase in Carbon or Nitrogen, or a decrease in Phosphorus in the bugs. The rate of chick growth, one variable that actually correlated significantly to soil quality, increased with more Phosphorus in the soil and decreased with more Carbon. I discovered that the number of chicks in a brood is positively correlated to the percent Phosphorus and negatively correlated to the percent Carbon of the bugs. The number of clutches produced in a year increases as Nitrogen increases or as Carbon decreases.
The sex ratio of a brood (proportion of males) can be manipulated by different species in various stages of development - from fertilization until the chick leaves the nest. I had hypothesized that a higher nutrient content (more Nitrogen and Phosphorus) would lead to a larger proportion of males because they are bigger and presumably need better nutrition. My results revealed, however, that there are more females when there is more Phosphorus and more males when there is more Carbon and Nitrogen in their diet.
For additional documentation Lori Simpers provided a PowerPoint Presentation entitled "Eastern Bluebirds: The Effect of Soil and Food Quality on the Health, Growth Rate, and Sex Ratio of Nestlings " provided here in PDF form.